1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus for medical diagnosis, and more particularly to a support structure for a gradient coil incorporated in the apparatus.
2. Description of the Related Art
MRI apparatuses are required to generate a very strong static magnetic field around a subject. A gradient field, which has its magnetism varied with time, is added to the static magnetic field. When a subject is exposed to a high-frequency magnetic field, the subject emits a high-frequency magnetic resonance signal. This signal is used to reconstruct a tomogram of the subject.
In general, the intensity of the static magnetic field must be set to about several kG to 10 kG (1T). Further, static magnetic field must have spatial uniformity, and generally needs a spatial uniformity of several tens of ppm or less. The spatial region required for a static magnetic field is, for example, a sphere with a diameter of 50 cm.
Recent MRI apparatuses are required to perform high-speed switching of their respective gradient fields and to increase the intensity of the fields in accordance with increases in the speed of processing in imaging techniques. MRI apparatuses may generate noise as a result of interaction of the current flowing through their gradient coils and the static magnetic field. This noise has been more and more increased under the above-described circumstances, and has reached 100 dB(A) or more. Accordingly, it is necessary to employ countermeasures against noise (such as the use of earplugs or ear defenders) to subjects of imaging.
Such noise includes air-propagated noise, and a solid-propagated noise transmitted through solid materials that contact the gradient coil. As general noise reduction methods, there is a method for surrounding a gradient coil with a sound absorbing or blocking member, and a method for sealing the gradient coil itself in a vacuum container in an airtight manner (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 63-246146, Jpn. Pat. Appln. KOKAI Publication No. 10-118043, or U.S. Pat. No. 5,793,210).
Further, to reduce the level of noise caused by solid-propagated sound, it has been attempted to reduce the rate of transmission of vibration to a magnet generating a static magnetic field, by supporting the gradient coil with vibration-proof rubber.
FIGS. 3A to 3C show the structure of a magnet-support-type magnet gantry employed in a conventional MRI apparatus.
The magnet gantry comprises a housing 1, and a magnet 2 for generating a static magnetic field and a gradient coil 3 contained in the housing 1. The gradient coil 3 is supported by the static field magnet 2 via support members 4a and 4b and damping members 5a, 5b, 5c, 5d, 5e and 5f. 
A bed 6 has a top board 61 on which a subject (not shown) is placed. When the top board 61 is moved, the subject is inserted through one opening of the gradient coil 3 into the internal space of the coil 3, or pulled out of the space through the one opening. The top board 61 is moved along the axis (Z axis) of the static field magnet 2 and gradient coil 3. The above-mentioned opening is used as an inlet/outlet for the subject. The above-mentioned opening is situated near the bed 6 when viewed from the magnet gantry.
The support members 4a and 4b are attached to the opposite ends of the static field magnet 2. In other words, the support members 4b and 4a are provided near and away from the bed, respectively.
The damping members 5a and 5b are provided between one end of the gradient coil 3 and the support member 4a and between the other end of the coil 3 and the support member 4b, respectively, and damp the Z component of vibration of the coil 3. The damping members 5c and 5d and damping members 5e and 5f are provided near and away from the bed, respectively. Further, as shown in FIGS. 3B and 3C, the damping members 5c, 5d, 5e and 5f are in contact with the periphery of the gradient coil 3 from obliquely below to damp the X- and Y-directional vibration of the gradient coil 3.
Thus, the support member 4b is a reflection of the support member 4a with respect to the center of the static magnetic field near and away from the bed. Similarly, the damping members 5a, 5c and 5d are a reflection of the damping members 5b, 5e and 5f with respect to the center of the static magnetic field near and away from the bed. As a result, the support members 4a and 4b support the gradient coil 3 in a non-coupled manner.
FIGS. 4A, 4B and 4C show the structure of the floor-support-type magnet gantry of a conventional MRI apparatus. In this structure, elements similar to those shown in FIGS. 3A to 3C are denoted by corresponding reference numbers, and no detailed description is given thereof.
The floor-support-type magnet gantry differs from the magnet gantry shown in FIGS. 3A to 3C in that the former employs support members 7a and 7b instead of the support members 4a and 4b. 
The support members 7b and 7a are provided near and away from the bed, respectively, and fix the opposite ends of the gradient coil 3 to the floor of the housing 1.
Like the support members 4a and 4b, the support member 7b is a reflection of the support member 7a with respect to the center of the static magnetic field near and away from the bed. Further, the damping members 5a, 5c and 5d are a reflection of the damping members 5b, 5e and 5f with respect to the center of the static magnetic field near and away from the bed. As a result, the support members 7a and 7b support the gradient coil 3 in a non-coupled manner.
In recent years, there is a demand for reducing the Z-dimension of the magnet gantry to improve the interior comfort of the apparatus, to reduce the length of the entire apparatus including the bed, and hence reduce the space required for installing the apparatus, or to reduce the distance to the center of the magnetic field to thereby reduce the distance of movement of a subject and accordingly enhance throughput.
However, in the magnet gantry constructed as above, since the support members and damping members Z-axially project from the side of the gradient coil 3 positioned near the bed, the distance L1 between the inlet of the magnet gantry and the center of the static magnetic field is inevitably great, which acts against satisfying the above demand.